Hi Andi73,
As you probably know the bottom row of knobs control a second oscilator that can be used to FM modulate (SMod) or AM modulate (DMod) the main oscillator. The speed control on the first row is the speed of this second oscillator and is set as a ratio of the speed of the main oscillator. The range is 1/8th the speed of the main oscillator (0.125xspd) to 8 times the speed of the main oscillator (8.0xspd).
When tempo mode is off, the sweep is smooth and you can get all values between 0.125 and 8.00, when the tempo mode is on the speed quantizes to increments of 1/8th, so the options are (1/8xspd, 2/8xspd, 3/8xspd, …, 7 6/8 x spd, 7 7/8 x spd, 8 x spd). Because the denominator is always in increments of 1/8, you know that every 8 times through the second oscillator the pattern between the main and second oscillator will repeat. In other words, with tempo on mode, you should be able to get regularly repeating patterns based on the tempo at all settings of speed.
The Slow speed is 1/8 the speed of the Fast speed for Rotary because that's the ratio for Factory Leslies. The Slow speed is 1/4 of the Fast speed for all other effects because it sounded better for those effects.
Dan
